We have demonstrated that human diploid fibroblasts (HDF) use both glucose and glutamine as primary energy sources for growth. The present research is to delineate the pathway of glutamine metabolism and its regulation under different growth conditions in HDF. It will be necessary to document the involvement of several enzymes (phosphoenolpyruvate carboxykinase, pyruvate carboxylase, malic enzyme) in glutamine metabolism. The level and cellular compartmentalization of these enzymes will be investigated. We have evidence that the early steps of gluconeogenesis, i.e., the conversion of 4-carbon to 3-carbon compounds, is very active in these cells. The effect of growth factors on this process will be studied in fibroblasts from controls, patients with lacticacidosis and infants dying from the Sudden Infant Death Syndrome. The latter have been proposed to be deficient in the enzyme phosphoenolpyruvate carboxykinase.